340 HP's vs headers

I know this may be a nutty question on this topic but here goes. I got a set of 68 hi po exhaust manifolds. My machinest and i discussed a 408 stroker, 6,000 rpm limit. Mostly low to midrange grunt, with a 4 speed O/D and 3.73 combo, eventual upgrade to a Passon 5 speed. I want it to be a sleeper looking like a stone stock 273 4 barrel. He is talking 9.5 to 1 CR for pump gas, and about 450-500 HP. He looked at me like i was nuts for wanting to extrude hone the 340 manifolds and use them. He told me choking it like that i'd be lucky to get 350HP out of it. I really dont want headers. I hate them actually. Will i really lost that much HP out of a stroker like that with 340 exhaust manifolds on it? If i lose 40-50 hp with that bottleneck, but its still grunting out a tick over 400-425 i'm ok with that. Whst he's describing is a 100hp loss. Any thoughts? Real world experience? I eould be nice to get a high figure doing a dyno pull and then say slap these 340s on and see what the drop really is. I bet these flow better than chivvy rams horns.